This invention relates to an electron energy filter and a transmission electron microscope provided with the same, and more particularly, relates to a device suitable for obtaining an elemental mapping image of a small area by separating only electrons having specific energy from an electron beam transmitted through a sample and by imaging it.
An electron energy filter is the device which resolves an electron beam transmitted through a sample and separates only electrons having energy in specific energy range, and mostly used in combination with a transmission electron microscope. Electrons transmitted through a sample lose energy which is inherent to constituent element of the sample due to inelastic scattering, therefore an electron microscopic image obtained with only electrons which lost specific energy represents the two dimensional mapping corresponding to constituent element of the sample. An electron beam containing wide energy range after transmission of a sample is filtered to allow only electrons with a specific energy range to pass, and resultant electron beam having restricted energy range images a contrasted image.
The electron energy filter is constituted with a plurality of magnetic poles comprising pole pieces which are provided facing each other with a certain gap, there is no magnetic field in a space between adjacent magnetic poles, the space is free spaces where an electron can move straight. Electrons entered into an energy filter along the center axis of an electron microscope pass through the gap between magnetic poles and the free space deviating from the center axis, then travel forward along the center axis again. An energy spectrum is obtained in the rear of the energy filter, and where specific energy is selected.
After the selection of energy, a two dimensional elemental mapping image is obtained using an imaging electron lens. The energy filter of this type is referred to as in-column type energy filter, various types such as .OMEGA.-type energy filter described in Japanese Patent Provisional Publication Showa-62-66553 (1987), .alpha.-type energy filter described in Japanese Patent Provisional Publication Showa-62-69456 (1987), and .gamma.-type energy filter proposed by the inventors of this invention have been disclosed.
To fabricate electron energy filters of these types, magnetic poles provided facing each other are required to be formed and positioned with high accuracy, a method of fabrication of these energy filters has been disclosed in Japanese Patent Provisional Publication Heisei-4-294044 (1992) for example.
This electron energy filter is referred to as .OMEGA.-type energy filter, which comprises a pair of outer plates and a pair of inner plates provided facing each other, and four magnetic poles for deflecting an electron beam are constituted with pole pieces which are provided facing each other with a certain gap in between.
Whole electron energy filter structured as described herein above is mounted in the vacuum chamber which is served as a column of the electron microscope.
Other structure of conventional energy filter (the second prior art) is described in, for example, Japanese Patent Provisional Publication Showa-58-32347 (1983), in the case of this energy filter, whole passage of electron beam in an energy filter is isolated using cylindrical non-magnetic material, and magnetic field is applied from the outside of the electron beam passage using sector magnetic poles.
In the above mentioned first prior art, whole energy filter is contained in a vacuum chamber and evacuated. In this case, all parts including the outer plates, inner plates, magnetic field exciting coils, magnetic poles, screws, and adjusting pins exist in a vacuum, therefore volume and surface area to be evacuated are very large. However, small electron beam passage holes provided at portions of entrance and exit for an electron beam are only the holes of the energy filter served as vacuum evacuation holes.
Because of these unfavorable conditions, it takes long time to evacuate the electron energy filter to a certain degree of vacuum, and gas released from coils adversely affects on the degree of vacuum and contaminates the sample. Especially when the surface morphology is observed and when an ice-embedded tissue sample is observed, the contamination of the sample is fatal. To obtain high vacuum, the evacuation under heating using a heater is required, but the heater is required to be contained in the vacuum internal in the case of this structure, therefore the evacuation under heating does not exhibits significant vacuum effect, this is a disadvantage of the first prior art.
Additionally for the first prior art described herein above, each pole piece is independent from an outer plate, these members are positioned with adjusting pins and screwed. Each plate is positioned each other with adjusting pins through an inner plate.
Each pole piece which constitutes each magnetic pole is positioned indirectly through each member, therefore, parallelism of each pole piece is represented by the total of the flatness of the plate and pole piece, parallelism between the plate and pole piece, and parallelism between the plates. Hence, the looseness of the adjusting pin and poor flat accuracy and flatness reduces the positioning accuracy and parallelism of the pole piece. On the other hand, the extremely high working accuracy and buildup accuracy are required to obtain the required positioning accuracy and parallelism, these conditions result in unfavorable cost.
On the other hand, in the second prior art described above, only the passage of electron beam is evacuated independently, therefore, the second prior art is not involved in the vacuum problem different from the first prior art. However, the passage of an electron beam used for the second prior art is made by welding tubes consisting of non-magnetic material which have been bent to fit the passage of electron beam or welding plates which are provided with grooves to fit the passage of electron beam. During the welding, the members are heated to cause partial distortion and magnetization depending on the material, the magnetization often causes the deviation of electron beam from the passage.
The welding buildup of the electron beam passage often causes vacuum leak. The electron beam passage is located in the gap between pole pieces and the tube of non-magnetic material is placed in the gap, such structure reduces the space available for the electron beam passage to a half. On the other hand, if the transmission of electrons is improved by increasing the inside diameter of the electron beam passage and the space between pole pieces, coil current should be increased in proportion to the space between magnetic poles to obtain required magnetic field, the increased coil current causes the drift due to heating of the coil and deviation of the electron beam position, these are disadvantages of the second prior art.